After three consecutive dry winters from 2015 to 2017, “Day Zero (https://www.nationalgeographic.com/science/article/partner-content-south-africa-danger-of-running-out-of-water)” — when the taps run dry — loomed large for Cape Town and its surrounds in 2018.

Municipalities introduced a slew of water restrictions and, almost overnight, Capetonians became familiar with greywater. Many previously let this untreated water from baths, showers, kitchen sinks, washing machines and the like run down the drain. Then they installed storage tanks or used buckets of it to flush their toilets or water their lawns. It was a boom time for domestic greywater technologies (https://www.nationalgeographic.com/science/article/partner-content-six-innovative-ways-water-was-saved).

Thanks to residents’ sparing use of water, including the adoption of greywater use, Day Zero never arrived. But there are lessons to be learnt from that experience, especially around the potential of greywater in seasons of plenty and drought...